Study on the biogas potential of anaerobic digestion of coffee husks wastes in Ethiopia

The poorly controlled discharge of coffee husks in Ethiopia causes severe environmental pollution and is a waste of resources. The volatile solid and carbon content in coffee husks waste indicates that it is rich in organic matter and has huge potential to produce biogas. This study investigated the feasibility of coffee husks to produce biomass through anaerobic digestion, based on temperature, initial pH, inoculum/substrate (I/S) ratio and carbon/nitrogen (C/N) ratio. The study demonstrated that the maximum production of biogas and methane reached 3359.6 ml and 2127.30 ml, respectively, under the conditions of mesophilic temperature (35±1°C), an initial pH of 7, an I/S ratio of 0.75 and a C/N ratio of 30. Based on this result, the effects of trace elements (Fe²⁺, Ni²⁺, Co²⁺) on biogas production and methane content were also explored. Compared with the group with no addition of trace elements, the experiment adding trace elements had significant enhancement effects on the production of biogas and methane, in which Fe²⁺ played a leading role (p<0.05). Fe²⁺ promoted the hydrolysis and acidification of coffee husks, resulting in the production of a series of intermediates such as volatile fatty acids and the other kinds of dissolved organic matter. Furthermore, the cooperation of Ni²⁺, Co²⁺ and Fe²⁺ enhanced the activity of the enzyme system in methanogens, promoting methane production. The results in this paper show that coffee husks have clear biogas potential through anaerobic digestion, and its effective utilization could fulfill the dual purpose of solid waste reclamation and local environmental protection in Ethiopia.